Чого ви хочете? Чаю чи кави? 'What do you want? Some tea or some coffee?' (this option assumes you don't offer all the thing, but just some part of it — so, it sounds OK for things that can be divided into parts)

If the thing cannot be divided into parts, and used as a whole, then accusative sounds better:

Що ви хочете? Компʼютер чи планшет? 'What do you want? A computer or a tablet?' (here, we're talking about items as a whole, so we use accusative)

Since we don't have the context in Duolingo, I believe both options should be accepted.